This invention relates to mobile communications systems, and particularly to means for determining handover in such a system.
One particular type of mobile communication system is a cellular radio telephone system. Cellular radio telephone systems generally include a switch controller coupled to the public switched telephone network (PSTN) and a plurality of base stations. One or more mobile stations communicate with a base station that facilitates a call between the mobile station and the PSTN. The communication link over a carrier signal from the base station to a mobile station is referred to as the downlink. Conversely, the communication link from a mobile station to the base station is referred to as the uplink.
A particular type of cellular radio telephone system is the GSM system (Global System for Mobile Communications), a description of which can be found in the book xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by M. Mouly and M. Pautet.
GSM networks generally include mobile services switching centres, base stations, and mobiles stations. Each of the plurality of base stations generally defines a geographic region or xe2x80x9ccellxe2x80x9d proximate to the base station to produce coverage areas.
As the mobile station moves from one cell to the next, the communication link is transferred from its current base station to a neighbouring base station using a procedure known as handover or handoff. The need for handover is usually determined on the basis of one or more criteria. Commonly used criteria are received signal strength indication (RSSI) of the mobile station at the base station, the distance from the current base station as determined by the round trip time for signals to and from the mobile station and bit error rate (BER).
There is a variety of methods by which a handover process can be initiated. Firstly, processes for initiating handover may make measurements on the uplink and downlink between the mobile station and the base station and initiate handover when received signal levels, received quality levels or the timing advance (based on the round trip time) values separately exceed thresholds. Alternatively, processes for initiating handover may compare measurements of received signal strength with received quality to deduce that bad quality may be the result of interference. Further, processes for initiating handover may additionally make measurements from a number of neighbour cells and initiate handover based on the relative level of the signal received from the serving cell and from the neighbour cells.
One feature which all of these known methods share, is that they only take into account whether or not a fixed threshold has been exceeded and do not take account of how much the threshold has been exceeded. As threshold levels for triggering a handover are usually set (at each base station) on a per cell basis, for the worst expected case, there can be situations where handovers are triggered before they are necessary. This leads to the disadvantageous effect of decreasing the call-handling capacity of a cell, and of running the risk of a handover xe2x80x9cping-pongxe2x80x9d effect where a call is repeatedly handed back and forth between cells unnecessarily. Conversely, in other situations, the triggering procedure takes too long and the call is dropped before handover can occur.
Another disadvantage of known methods is the complexity of the handover process. For example, four parameters need to be set in a database at the base station in order to manage a handover based on a quality measurement.
These parameters are hreqave (the number of received quality measurement samples to be averaged), hreqt (buffer size), xe2x80x9cnxe2x80x9d and xe2x80x9cpxe2x80x9d, where xe2x80x9cpxe2x80x9d is the number of xe2x80x9cnxe2x80x9d consecutive averaged values which need to cross a set threshold in order to trigger a handover.
Thus there is a need for a means for triggering a handover which mitigates the above disadvantages.
In a first aspect, the present invention consists of a device for determining handover in a mobile communications system and comprises:
an averager for receiving measurements of a communications signal parameter and for computing an average of said measurements;
a first comparator for detecting when said average exceeds a first threshold;
a summer for computing a cumulative sum of averages exceeding the first threshold; and
a second comparator for detecting when the cumulative sum exceeds a second threshold, thereby to generate a handover trigger signal.
The device may be incorporated in a base station or in a mobile station.
In a second aspect, the present invention consists of a method for determining handover in a mobile communications system including the steps of:
receiving measurements of a communications signal parameter and computing an average of said measurements;
determining if an average exceeds a first threshold;
summing those average values which exceed the first threshold to produce a cumulative sum; and
determining if the cumulative sum exceeds a second threshold, thereby facilitating generation of a handover trigger signal.
The signal parameters may comprise, for example, quality, interference, received RF level or measurements relating to the distance between the base station and the mobile station, i.e., the timing advance.
The summer may apply a weighting factor to the average values exceeding the first threshold depending upon the type of parameter being used.
The invention is applicable to macro cell, micro cell and pico cell configurations, for example.
Further, it is not limited to use solely with the GSM cellular system.
The invention thus takes into account the variations in received signal level, thereby facilitating self-adaptive handovers. Handovers can be controlled to be as fast or as slow as needed.
Another advantage is that of simplification because handover parameters can be defined at a network level rather than a cell level. Further, only one measured parameter need be used for the entire network along with xe2x80x9chreqavexe2x80x9d, the number of samples to be averaged, thus reducing the complexity compared with the known systems.
In particular, for handovers based on quality measurements, micro cells (i.e. lower layer cells) can support more calls because the relevant parameters do not need to be set up for the worst case. Instead, handovers are triggered depending on the degree of quality degradation i.e. the worse the quality, the quicker the handover. Thus, call drop rate is reduced.